1. Field of Invention
This invention relates to surface acoustic wave (SAW) sensors, such as pressure sensors, and more particularly to an improved SAW pressure sensor structure.
2. Description of the Prior Art
SAW pressure sensors are known in the art, as reported in U.S. Pat. Nos.3,978,731 and 4,100,811. The SAW pressure sensors are provided by drilling into one surface of a SAW delay line substrate to form an interior, cylindrical cavity in the substrate, such that a deformable diaphragm is formed between a first surface of the SAW delay line and the interior surface formed in the end wall of the cavity. When stress is applied to the diaphragm, such as by an applied fluid pressure, the diaphragm deforms causing a change in the acoustic wave propagation characteristics in that portion of the SAW delay line forming the diaphragm. The change in acoustic wave propagation velocity is measured as a change in the frequency of oscillation of external oscillator circuitry connected in series with the SAW delay line regenerative feedback loop. The operating frequency changes as a result of changes in the propagation velocity, all of which is described in detail in the hereinbefore referenced U.S. patents.
The process of forming the diaphragm in the SAW substrate by boring or drilling a central cavity is subject to a number of limitations. The sensitivity of the pressure sensor is determined in part by the thickness of the deformable diaphragm and it is desirable to accurately control the diaphragm thickness. The depth of the cavity is difficult to control during the drilling process, such that the diaphragm membrane may not always be provided at the optimum thickness. Attempts at providing thin diaphragms often lead to a breakout into the first surface of the substrate. In addition, it is also essential in sensitive devices to maintain parallelism of the diaphragm membrane which is difficult in the drilling process. The drilling itself many times results in microcracks in the interior cavity walls around the periphery of the diaphragm, which in many instances have resulted in device failure due to a ruptured diaphragm at higher operating pressures. Also the diaphragm formed by the cavity has one polished surface (that of the SAW substrate surface containing the delay line) and one unpolished, quasi-lapped surface provided by the ultrasonic boring. creating the possibility of a loss of sensitivity across the diaphragm by allowing the creation of possibly different strains on either surface of the diaphragm.
All of these difficulties in fabrication of the diaphragm may be overcome by building a SAW sensor in two pieces, i.e. two substrates, as suggested in the aforementioned U.S. Pat. No. 3,978,731. There it is disclosed that a separate diaphragm which is either a metal, a dielectric, or a semiconductor material, is bonded to a substrate through ". . . the use of glue, solder, weld, thermo-compression or other joining techniques, . . .". Although there is a lack of disclosure as to the bonding process it may be assumed that the bonds recited may be provided through techniques known in the art. Each of the bonds recited, however, result in limited operating sensor characteristics for any dual substrate sensor formed thereby. The use of a glue presents an obvious limitation on operating temperature. Temperatures which do not exceed, but are close to, the melting temperature result in mobility between the diaphragm and the substrate. In addition the glue is organic and presents a dirty element in any vacuum environment in which the SAW sensor is enclosed, due to the inherent outgassing characteristic of the glue. From either a temperature operating standpoint, or from the standpoint of providing a good vacuum environment for the SAW sensor, the use of a glue bond is unsatisfactory. The other recited bonds, i.e. solder, weld or thermo-compression, all involve the use of metallization between the diaphragm and substrate. Although this may not present a problem where, as recited in the patent, the diaphragm itself is metal, it prevents a severe problem of induced strain into a piezoelectric diaphragm and substrate. The metal seal being adjacent to the diaphragm, any temperature cycling of the SAW sensor provides induced strain into the diaphragm causing severe degradation in the sensitivity of the SAW device. Therefore, all of the recited bonds, both glue and metal, are unsatisfactory for providing a SAW sensor having piezoelectric diaphragm, although any or all of the bonds may have application with either metal diaphragm devices or with low operating temperature devices.